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Special Issue Information

Dear Colleagues,

The advent of controlled polymerization techniques in the last few decades has truly revolutionized synthetic polymer chemistry. With the contemporary toolbox of synthetic methods at hand, a large variety of materials can be accessed and the frontier in research has been shifted from the pure design of complex macromolecular structures towards materials with biological precision. Macromolecules can contain encoded information via sequence control (e.g., sequences of small polydisperse segments of true monomer sequences) or can exert complex behaviors, such as chain folding, the formation of tertiary structures or generally, the formation of complex single-chain nanoparticles. Development of such materials requires highly precise methods in synthesis, in-depth studies on the physical properties of the macromolecules, and high-level characterization of products. Even though precise polymer synthesis is still a young field of exploration, developments are fast and new generations of synthetic materials are rapidly evolving and are becoming closer and closer in structure, shape, and function to biomacromolecules.

This Special Issue focuses on the latest achievements in the field of precision polymer synthesis with respect to the development of polymeric materials that match or advance biological precision and function. Therefore, we will highlight topics concerning the synthesis of complex polymers, their physical properties, their ability to fold and arrange themselves, and the characterization of such materials with state-of-the-art methodologies.

Prof. Dr. Thomas JunkersGuest Editor

Submission

Manuscripts should be submitted online at www.mdpi.com by registering and logging in to this website. Once you are registered, click here to go to the submission form. Manuscripts can be submitted until the deadline. Papers will be published continuously (as soon as accepted) and will be listed together on the special issue website. Research articles, review articles as well as communications are invited. For planned papers, a title and short abstract (about 100 words) can be sent to the Editorial Office for announcement on this website.

Submitted manuscripts should not have been published previously, nor be under consideration for publication elsewhere (except conference proceedings papers). All manuscripts are refereed through a peer-review process. A guide for authors and other relevant information for submission of manuscripts is available on the Instructions for Authors page. Polymers is an international peer-reviewed Open Access monthly journal published by MDPI.

The effect of the irreversible addition-fragment chain transfer agent, butyl(2-phenylallyl)sulfane (BPAS), on the course of the emulsion polymerization of styrene and on the product molecular weight was investigated. The emulsion polymerizations were performed using various amounts of sodium dodecyl sulfate (SDS) as the

The effect of the irreversible addition-fragment chain transfer agent, butyl(2-phenylallyl)sulfane (BPAS), on the course of the emulsion polymerization of styrene and on the product molecular weight was investigated. The emulsion polymerizations were performed using various amounts of sodium dodecyl sulfate (SDS) as the surfactant and potassium peroxodisulfate (KPS) as the initiator. The relationships between the rates of polymerization (\(R_{p} \)) and the number of particles per volume (\(N_{c} \)) with respect to the concentrations of KPS, SDS, and BPAS were found to be \(R_{p} \propto \left\lbrack KPS \right\rbrack^{0.29} \), \(N_{c} \propto \left\lbrack KPS \right\rbrack^{0.26} \),\(R_{p} \propto \left\lbrack SDS \right\rbrack^{0.68} \), \(N_{c} \propto \left\lbrack SDS \right\rbrack^{0.72} \), and \(R_{p} \propto \left\lbrack BPAS \right\rbrack^{- 0.73} \) . The obtained relationships can be attributed to the exit of the leaving group radicals on BPAS from the polymer particles. The experimental values of the average number of radicals per particle (\(\overset{\_}{n} \)) were strongly dependent on the BPAS concentration and were in good agreement with the theoretical values (\({\overset{\_}{n}}_{theo} \)) from model calculations. The number-average molecular weight (\(\overset{\_}{M_{n}} \)) can be controlled by BPAS over nearly the entire conversion range, which is also in agreement with the mathematical model. In addition, the transfer rate coefficient (\(k_{tr} \)) of BPAS can be estimated as 326 L/mol/s at 70 \(^\circ\)C. Moreover, similar good results were found for the tested redox reactions at 30 \(^\circ\)C.
Full article

A novel well-defined linear poly(ε-caprolactone) (P1) containing terminal azobenzene and ethyne groups was successfully synthesized through tin-catalyzed ring-opening polymerization of ε-caprolactone in the presence of N,N′-bis(2-hydroxyethyl)-4-(3-ethynylphenylazo)aniline (BHA) in bulk. Subsequent reactions allowed the synthesis of the corresponding bromoester end-functionalized polymer (P2), which was converted into AB2 type polymer (P3) containing terminal azide groups with NaN3. Consequently, hyperbranched poly(ε-caprolactone) (HPCL) was prepared with AB2 macromonomer (P3) by “click” chemistry under the catalysis of CuSO4·5H2O/sodium ascorbate/H2O. The structure of the resultant HPCL was characterized by gel permeation chromatography (GPC), proton nuclear magnetic resonance (1H-NMR), ultraviolet-visible (UV-Vis) spectroscopy and fourier transform infrared spectroscopy (FT-IR). Thermal and crystallization properties of P1 and HPCL were further studied by differential scanning calorimetry (DSC), wide-angle X-ray diffraction (WAXD) and polarised optical microscopy (POM). These results indicated that the crystallinity of HPCL was slightly lower than that of P1 due to the hyperbranched structure of HPCL. Additionally, the photo-induced trans-cis isomerization behaviors of BHA, P1 and HPCL containing terminal azobenzene were investigated in chloroform solution, and the photoisomerization rate constant (kexp) of small molecule (BHA) was nearly three times faster than that of polymers P1 and HPCL, which was due to the sterically hindering effect of the polymer-chain configuration.
Full article

In high-conversion atom transfer radical polymerization (ATRP), all the reactions, such as radical termination, radical deactivation, dormant chain activation, monomer propagation, etc. could become diffusion controlled sooner or later, depending on relative diffusivities of the involved reacting species. These diffusion-controlled reactions directly affect the rate of polymerization and the control of polymer molecular weight. A model is developed to investigate the influence of diffusion-controlled reactions on the high conversion ATRP kinetics. Model simulation reveals that diffusion-controlled termination slightly increases the rate, but it is the diffusion-controlled deactivation that causes auto-acceleration in the rate (“gel effect”) and loss of control. At high conversions, radical chains are “trapped” because of high molecular weight. However, radical centers can still migrate through (1) radical deactivation–activation cycles and (2) monomer propagation, which introduce “residual termination” reactions. It is found that the “residual termination” does not have much influence on the polymerization kinetics. The migration of radical centers through propagation can however facilitate catalytic deactivation of radicals, which improves the control of polymer molecular weight to some extent. Dormant chain activation and monomer propagation also become diffusion controlled and finally stop the polymerization when the system approaches its glass state.
Full article

A systematic investigation into the chain transfer polymerization of the so-called radical precursor polymerization of poly(p-phenylene vinylene) (PPV) materials is presented. Polymerizations are characterized by systematic variation of chain transfer agent (CTA) concentration and reaction temperature. For the chain transfer constant, a negative activation energy of −12.8 kJ·mol−1 was deduced. Good control over molecular weight is achieved for both the sulfinyl and the dithiocarbamate route (DTC). PPVs with molecular weights ranging from thousands to ten thousands g·mol−1 were obtained. To allow for a meaningful analysis of the CTA influence, Mark–Houwink–Kuhn–Sakurada (MHKS) parameters were determined for conjugated MDMO-PPV ([2-methoxy-5-(3',7'-dimethyloctyloxy)]-1,4-phenylenevinylene) to α = 0.809 and k = 0.00002 mL·g−1. Further, high-endgroup fidelity of the CBr4-derived PPVs was proven via chain extension experiments. MDMO-PPV-Br was successfully used as macroinitiator in atom transfer radical polymerization (ATRP) with acrylates and styrene. A more polar PPV counterpart was chain extended by an acrylate in single-electron transfer living radical polymerization (SET-LRP). In a last step, copper-catalyzed azide alkyne cycloaddition (CuAAC) was used to synthesize block copolymer structures. Direct azidation followed by macromolecular conjugation showed only partial success, while the successive chain extension via ATRP followed by CuAAC afforded triblock copolymers of the poly(p-phenylene vinylene)-block-poly(tert-butyl acrylate)-block-poly(ethylene glycol) (PPV-b-PtBuA-b-PEG).
Full article

Planned Papers

The below list represents only planned manuscripts. Some of these
manuscripts have not been received by the Editorial Office yet. Papers
submitted to MDPI journals are subject to peer-review.

Title: Studies of Styryl-TITNO for use in syntheses of butyl acrylate/styrene block copolymers by NMP at temperatures < 100 °CAuthor: Peter A. Lovell (corresponding), Neil R. Cameron and Bencha ThongnuanchanAbstract: The feasibility of using 2,2’,5-trimethyl-3-(1-phenylethoxy)-4-tert-butyl-3-azahexane (Styryl-TITNO) to synthesize n-butyl acrylate / styrene block copolymers via sequential nitroxide-mediated polymerization is demonstrated. Optimised conditions have been established, with a high degree of livingness, and involve formation of the first poly(n-butyl acrylate) block at 90 °C, followed by growth of the second polystyrene block at 70 °C. This correlates well with the optimised conditions for the corresponding homopolymerisations reported in Polymer 55 (2014) 772-781.